In video and image processing, the size of an object in a video image sequence is a key descriptor for the object. In addition, detection and estimation of a size change of the object is informative to many applications. The estimation of the size change leads to a scaling ratio of the object. For example, in a camera surveillance system, a size change of a person appeared on the video image captured by a camera indicates that the person comes closer or go away from the camera. A scaling ratio of the person appeared on the video frames captured on different times can be used to calculate the distance of the person from the camera. Knowing this distance enables tracking the person. There is a need to estimate the scale factor of an object in a video image sequence, preferably making use of motion information in the form of motion vectors commonly present in the video image sequence.
In US20030122853, US20080049145, U.S. Pat. No. 6,115,067, US20080036867 and US20080291271, a displacement calculated from a motion vector is used to detect if there is any movement of an object. However, a method to calculate the scale ratio is not given.
A method for evaluating a scale factor (i.e. a scale ratio) is disclosed in U.S. Pat. No. 6,549,683. The scale factor is determined by comparing a scaled image with a reference template. The need to construct a reference template, however, is typically not a simple task.
In US20090245755, blocking boundary artifact pixels are detected in the two decoded images to indicate the object boundaries it then computes a sum of pixel values for each blocking boundary artifact and the pixel distance between each pair or neighboring block boundaries. The scaling ratio is determined based on the distance value and the sum of pixel values. However, blocking boundary artifact does not always correspond to the actual object boundary. Besides, the application of this invention is limited to the decoded images only.
In US20060126737, a scaling ratio is calculated through measuring the displacements in the x-direction and in the y-direction independently based on the motion vectors. Although the scaling ratio of an object having a rectangular shape can be estimated with accuracy by the aforementioned approach, a high accuracy may not be achieved in case the object has an irregular shape.
There is a need in the art to have an improved, simple method for estimating the scaling ratio, particularly if the object has an irregular shape.